Our invention relates to electric inductive apparatus such as transformers, reactors, and the like. The invention is directed to high voltage coils of the disc winding type and particularly to the use of electrostatic shields in such coils for improving the voltage distribution and thereby reducing the insulation stresses created by the application of steep wave front or impulse voltages to these disc winding type coils.
It is well known that highly inductive windings such as iron core transformer and reactor windings, when exposed to steep wave front or surge voltages, initially exhibit an expotential distribution of voltage drop along the length of the coil winding with a very high voltage gradient at the first few turns adjacent the line terminal or high voltage end of the coil. This non-uniform distribution of surge voltages or potentials is undesirable, as it necessitates thicker insulation between the conductor turns of the coils, and thicker insulation between the first few coil sections adjacent the line terminal. Size and cost of electrical inductive apparatus is thus adversely affected. Merely increasing the thickness of electrical insulation does not insure that the winding will not fail when subjected to surge potentials, as increasing the thickness of the electrical insulation reduces the internal or series capacitance of the turns and coil sections at the line end, which causes a still more unfavorable distribution of surge potential. This condition arises because the winding presents an impedance which is predominantly capacitive to steep wave front voltages. Such capacitive impedance is made up of a complex network of capacitances in series and parallel circuit relation. If series capacitance only were present, voltage distribution throughout the winding would be substantially uniform and linear. The initial impulse voltage distribution of a transformer winding grounded at one end is given by the relation: ##EQU1## WHERE V.sub.o = Voltage applied to high potential terminal of coil winding.
X = percent distance along winding from line end. ##EQU2## C.sub.g = Total capacitance between the winding and ground plane. AND PA1 C.sub.s = total series capacitance of the winding.
This initial distribution is shown in FIG. 1. It can be seen from this figure that the voltage stress at the impulsed or line end of the coil, as represented by the curved line designated initial impulse voltage distribution, is greater than the steady state voltage distribution given by the straight line designated final distribution. It can be shown that this increase in stress is directly proportional to ".alpha.". It is therefore possible to lower this initial stress by reducing ".alpha.", which can be accomplished by increasing C.sub.S. It is therefore desirable to construct a winding in such a way that the series capacitance is large relative to the parallel or ground capacitance in the disc/coil winding.
Various electrostatic shielding arrangements have been utilized in the past in an attempt to increase the series capacitance in order to improve the initial distribution of an impulse voltage applied to coils of the aforementioned type. These arrangements have met with varying degrees of success and have resulted in either less than adequate performance or an unavoidable increase in coil size and/or cost. One arrangement illustrated in U.S. Pat. No. 2,905,911 to KURITA involves, in one embodiment thereof, the placing of uninsulated shield conductors between at least the two outermost conductor turns of each disc coil layer or coil section and then the connecting together of these shield conductors to form floating shield pairs. While this improves the initial impulse voltage distribution along the outside of the coil winding it does very little to improve the initial voltage distribution along the inside of the coil winding. Another arrangement illustrated in U.S. Pat. No. 3,380,007 to ALVERSON et al, involves in one embodiment thereof, the placement of shield conductors between the two outermost and the two innermost turns of each disc coil layer or coil section. The shields are connected in pairs, alternately along the inside and the outside of the coil windings, to the current carrying conductor at the opposite end of the disc coil section in which said shield is located, said connections to the current carrying conductors causing an undesirable increase in coil size and cost.
In order to avoid these and other disadvantages it would be desirable to provide an electrostatic shielding arrangement that would improve the initial distribution of impulse voltage applied to coil windings having serially connected disc type coil sections and in addition, reduce both the size and cost of adding such a shielding arrangement to such coils.
Accordingly, it is the principal object of the present invention to provide an electrostatic shielding arrangement that will improve the initial distribution of impulse voltage applied to coil windings having serially connected disc type coil sections.
Another object of the present invention is to provide an electrostatic shielding arrangement for disc type coil windings that will contribute a minimum increase to overall coil size.
A further object of the present invention is to provide an electrostatic shielding arrangement for disc type coil windings that will add a minimum amount of cost to the construction of such coils.